Semiconductor package and method of attaching semiconductor dies to substrates

ABSTRACT

A method of mounting a semiconductor die on a substrate with a solder mask on a first surface includes placing a die on the solder mask, and mounting the die to the substrate by applying pressure and heat. The applied pressure ranges from a bond force of approximately 5 to 10 Kg, the heat has a temperature range from approximately 150 to 200° C. and the pressure is applied for a range of approximately 1 to 10 seconds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application No.61/016,863 filed on Dec. 27, 2007, the disclosure of which isincorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of Invention

Apparatuses consistent with the present invention relate tosemiconductor packages and methods for manufacturing semiconductorpackages. More particularly, the present invention relates to asemiconductor package wherein a solder mask adheres a semiconductor dieto the substrate through the application of heat and pressure.

2. Description of the Related Art

Current methods for manufacturing semiconductor packages, such aslead-on-chip (LOC) devices, include applying a printable paste, epoxy ortape onto a substrate and then pressing the semiconductor die onto thepaste. Next, curing is performed (not if tape is used), which securelybonds the die to the substrate. The curing step can take up to threehours.

However, because the manufacture of semiconductor packages is a highvolume process, designers are always looking for ways to reduce cost andmanufacturing time. One cost savings approach that was investigated wasto reduce the volume of paste used in the current process, such as aknown as 4-dot printing design whereby four “dots” of adhesive areprovided on the substrate for die attachment. However, this approach wasunsuccessful because delamination of the die from the substrateoccurred.

Therefore, an objective of the present invention is to develop a methodof adhering a die to a substrate without the use of paste or adhesivetape. By eliminating the printable paste process, material and manpowersavings can be obtained and manufacturing time can be shortened throughthe elimination of the LOC cure.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention overcome the abovedisadvantages and other disadvantages not described above. Also, thepresent invention is not required to overcome the disadvantagesdescribed above, and an exemplary embodiment of the present inventionmay not overcome any of the problems described above.

An embodiment of the inventive method of mounting a semiconductor die ona substrate with a solder mask on a first surface includes placing a dieon the solder mask, and mounting the die to the substrate by applyingpressure and heat.

In another embodiment of the inventive method, the applied pressureranges from a bond force of approximately 5 to 10 Kg.

In another embodiment of the inventive method, the heat has atemperature range from approximately 150 to 200° C.

In another embodiment of the inventive method, the pressure is appliedfor a range of approximately 1 to 10 seconds.

In another embodiment of the inventive method, the heat causes thesolder mask to adhere said die to the substrate.

In another embodiment of the inventive method, the applied pressure is abond force of approximately 5 Kg.

In another embodiment of the inventive method, the heat has atemperature of approximately 175° C.

In another embodiment of the inventive method, the pressure is appliedfor approximately 5 seconds.

In another embodiment of the inventive method, the solder mask is notpre-baked before the die is attached.

An embodiment of the inventive semiconductor package includes asemiconductor die, a substrate, and a solder mask, wherein the soldermask adheres the die to the substrate.

In another embodiment of the inventive package, the substrate alsoincludes a core and a metal layer wherein the metal layer is between thecore and the solder mask.

In another embodiment of the inventive package, the solder mask is madeof acrylic epoxy.

In another embodiment of the inventive package, the solder mask has athickness of approximately at least 30 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or other aspects of the present invention will becomeapparent and more readily appreciated from the following description ofthe exemplary embodiments, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view of an embodiment of the inventivesubstrate with solder masks.

FIG. 2 is a table of technical specifications of an exemplary substrate.

FIG. 3 is a cross-sectional view of an exemplary mounting device.

FIGS. 4A, 4B, 5, 6 and 7 are tables showing results of different studiesrelated to the inventive method.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings.

Prior to arriving at the present invention, a baseline study wasperformed to aid in the determination of the optimum parameters andmethods. The study involved attempting to mount a plurality of dies to abare substrate. After the dies were placed on the substrate, variouspressures in the form of bonding forces (1 kg to 10 kg) and heat wereapplied. The temperatures ranged from 100° C. to 475° C. Afterapplication of the various pressures and heat, none of the dies adheredto the substrate. In addition, the substrate began showing signs ofburning at temperatures above 300° C. Burning was very apparent attemperatures at and above of 350° C. Also, the passivation coating ofthe die surface was damaged during the process.

Next, studies were performed to see if a solder mask could be used toadhere the die to the substrate. The studies used a two-metal layersubstrate composition supplied by Kinsus that had a solder mask appliedover the bare metal layers. The invention is not limited to substratessupplied by Kinsus. In this particular embodiment, the solder mask wasmade from acrylic epoxy. FIG. 1 shows a substrate with top and bottomsolder masks applied on the metal layers M1 and M_Base. The metal layerscover a substrate core. Technical specifications of an exemplarysubstrate are shown in FIG. 2. The solder mask may be applied to themetal layers by well-known printing methods. In addition, the soldermask is formed at least on areas between the die and the substrate.

In the particular embodiment shown in FIG. 1, the core is approximately200 μm thick, the metal layers are approximately 8-16 μm thick, and thesolder masks are approximately 30 μm thick. The total substratethickness, with the solder masks is approximately 285±50 μm. Soldermasks that are greater than 30 μm thick will also be acceptable for thisinvention.

FIG. 3 is a representation of a mounting device that can used to mountthe die to the substrate, with the solder masks already applied. In thestudy, a Renesas CM 700 series lead-on-chip mounter was used along withdiamond coated 8.0×8.0 mm mount and stage tools. The invention is notlimited to mounting machines supplied by Renesas. The die is positionednext to the corresponding substrate by the stage tool. After the stagetool and mount tool are heated, the mount tool presses the die andsubstrate together. The heat causes the material state of the soldermask to change, which causes the die to adhere to the substrate.

During the study, several combinations of temperature (e.g., 150, 175and 200° C.), bond force (e.g., 1, 2, 5 and 10 kg) and bond time (e.g.,1, 5 and 10 sec.) were used.

FIGS. 4A and 4B shows some of the different combinations and results. Inthe first six legs in FIG. 4A, none of the dies adhered to thesubstrate. Likewise, in the first leg in FIG. 4B, the die did notadhere.

In legs 7A and 7B in FIG. 4A, some dies adhered, however, approximatelyone-third of the dies did not adhere.

In legs 8A and 8B in FIG. 4A, all dies adhered satisfactorily. Theparameters that were used in stages 8A and 8B were a temperature of 200°C., pressure of 10 kg and a bond time of 10 seconds.

In legs 2 through 9 in FIG. 4B, the dies adhered satisfactorily, despiteissues with low die shear, high bond forces and high temperatures. Theoptimal combination of parameters occurred in leg 5. In that leg, thetemperature was 175° C., bond force was 5 kg and bond time of 5 seconds.

Next, studies were performed to see if oven baking impacted the process.Studies were carried out using substrates with solder masks that werenot pre-baked, and substrates with solder masks that were pre-baked. Asshown in FIG. 5, when the substrates that were not oven baked before dieattachment, all of the samples had acceptable results. However, when thesubstrates were oven baked before die attachment, many of the samplesfailed. Hence, the studies show that it is desirable to use substrateswith solder masks that are not pre-baked before assembly.

FIG. 6 is a summary of studies that were performed with differentconditions on the mounting device. As mentioned above, the mountingdevice used in these series of studies is Renesas CM 700 serieslead-on-chip mounter together with diamond coated 8.0×8.0 mm mount andstage tools. The mounting device has “pre-heat”, “pre-bake” or“laminate” functions typically designed for die attachment to substratesusing adhesive paste or tape. For example, the “pre-heat” function heatsup the adhesive before die attachment, the “pre-bake” function bakes theadhesive to remove moisture before die attachment and the “laminator”function laminates the adhesive (in the form of tape) onto the substratebefore die attachment. As the present invention does not utilize anyform of adhesive between the die and substrate, the inventors haveexperimented with these functions for die attachment to the solder masklayer of the substrate. The mounting device was configured asfollows—Legs A & B with pre-heat, pre-bake and laminator functions OFF,Legs C & D with pre-heat and pre-bake functions ON and laminatorfunction OFF, and Legs E & F with pre-heat, pre-bake and laminatorfunctions ON. It was found that die adhesion is most satisfactory inLegs A & B when the pre-bake, pre-heat and laminator functions are OFF.Die adhesion was also observed for Legs C & D but with lower die shearvalues. Some of the dies did not adhere in Legs E & F thereby showingthat conditions of pre-heat, pre-bake and laminator functions intogetherness are not desirable.

FIG. 7 shows additional test results.

In summary, the studies showed that good die to substrate adhesion ispossible when using the following parameter settings: Bond Force of 5-10kg; Bond Time of 1-10 seconds; Bond Temperature of 150-200° C. Optimalparameter settings would be Bond Force of 5 kg; Bond Time of 5 seconds;Bond Temperature of 175° C. The studies also show that substrates withsolder mask which are not pre-baked are desirable in this invention.

These parameter settings have been shown to provide good die tosubstrate adhesion even after MSL 3 testing. In addition, no die crackswere observed during the mounting process. Advantageously, the presentinvention provided means of attaching a die to a substrate without usingan adhesive paste or tape thereby resulting in process time, materialand manpower savings

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A method of mounting a semiconductor die on a substrate with a soldermask on a first surface comprising: placing a die on said solder mask;mounting said die to said substrate by applying pressure and heat. 2.The method of claim 1 wherein said applied pressure ranges from a bondforce of approximately 5 to 10 Kg.
 3. The method of any one of claims 1and 2 wherein said heat has a temperature range from approximately 150to 200° C.
 4. The method of any one of claims 1 and 2 wherein saidpressure is applied for a range of approximately 1 to 10 seconds.
 5. Themethod of claim 1 wherein said heat causes said solder mask to adheresaid die to said substrate.
 6. The method of claim 1 wherein saidapplied pressure is a bond force of approximately 5 Kg.
 7. The method ofany one of claims 1 and 6 wherein said heat has a temperature ofapproximately 175° C.
 8. The method of any one of claims 1 and 6 whereinsaid pressure is applied for approximately 5 seconds.
 9. The method ofclaim 1 wherein said solder mask is not pre-baked before said die isattached.
 10. A semiconductor package comprising: a semiconductor die; asubstrate; and a solder mask; wherein said solder mask adheres said dieto said substrate.
 11. The semiconductor package of claim 10, whereinsaid substrate further comprises a core and a metal layer wherein saidmetal layer is between said core and said solder mask.
 12. Thesemiconductor package of claim 10, wherein said solder mask comprisesacrylic epoxy.
 13. The semiconductor package of claim 10, wherein saidsolder mask has a thickness of approximately at least 30 μm.